THE DETERMINATION OF THE BARRIER TO INTERNAL ROTATION IN NITROUS ACID BY MATRIX ISOLATION

Abstract

“Nitrous acid has been produced by photolysis of mixtures of $HN_{2}$ and $oxygen.^{1,2}$ The HONO is isolated in matrices of solid argon and nitrogen at $20^{\circ} K$ and examined by infrared $techniques.^{3}$ The assignment of observed frequencies to cis- and trans- isomers of HONO is straightforward since there is little interference from other oxides of nitrogen and water, and because of the absence of rotational structure in the matrix isolation spectra. The assignments of cis- and trans- HONO are in general agreement with the published gas phase $work.^{4,5}$ It is possible to measure the rate of change of the cis- to the more stable trans- form of HONO at $20^{\circ} K$. This rate is reasonably consistent only with a quantum-mechanical tunneling mechanism for the isomerization. The effect of isotopic substitution on the tunneling process offers confirmation of the proposed mechanism. The rate of tunneling is proposed as a sensitive method of establishing the heights of barriers to internal rotation for assumed barrier shapes.”